The present invention relates to a process for the continuous production of superabsorbent polymers based on fine-particle, uncrosslinked and crosslinked aqueous polyacrylonitrile emulsions.
Superabsorbent polymers are known and are principally employed in the production of diapers and incontinence articles. These polymers are also used as water-storing materials in agriculture and also for the sheathing of electrical cables. As a rule, the commercially available superabsorbent polymers are wide-mesh-crosslinked, water-insoluble polymers based on alkali salts of polyacrylic acid or based on copolymers of acrylic acid and acrylamide which are obtained by radically initiated co-polymerisation of acrylic acid and polyfunctional monomers such as, for example, divinylbenzene, ethylene glycol dimethacrylate, ethylene glycol diallyl ether, butane-diol acrylate, hexanediol methacrylate, polyglycol diacrylate, trimethylolpropane diacrylate, allyl acrylate, diallylacrylamide, triallylamine, diallyl ether, methylene-bisacrylamide and N-methylolacrylamide. By reason of their molecular structure, such polymers are capable of absorbing large amounts of liquids, accompanied by swelling and formation of hydrogels, and of retaining said liquids even under pressure.
Superabsorbent polymers with extremely high swelling power and high gel strengths are described in patent applications EP-A-670 335 and EP-A-697 416. These products are obtained by alkaline hydrolysis of polyacrylonitrile (PAN) emulsions at 50-100xc2x0 C. and at reaction-times of 1 to 2 hours. With this process, products having superabsorbent properties are isolated after hydrolysis in the form of fine-particle powders by precipitating with solvents such as aliphatic monoalcohols, for example. After filtration and drying, the superabsorbent polymers are ground to the desired particle size.
The fine-particle, aqueous, high-molecular, uncrosslinked or crosslinked polyacrylonitrile emulsions that are required for production of the superabsorbent polymers are obtained by the homopolymerisation and/or copolymerisation of acrylonitrile in the presence of special anionic polymeric emulsifiers (see patent applications EP-A-331066 and also EP-A-590 460). The molecular weights of the uncrosslinked polyacrylonitrile emulsions that are prepared by this process lie within the range from 5-105 to 1-107 g/mol. The particle sizes of the uncrosslinked or crosslinked aqueous PAN emulsions lie within the range between 100 and 300 nm (determined by means of laser correlation spectroscopy).
In the course of the hydrolysis of such PAN emulsions with aqueous solutions of alkali hydroxides the partially hydrolysed homopolymers and/or copolymers of acrylonitrile are formed, in which 30 to 80 mol. % of the nitrile groups have been transformed into carboxylate groups and 20 to 70 mol. % of the nitrile groups have been transformed into carboxamide groups and 0 to 20 mol. % of the nitrile groups remain unchanged.
By reason of the transition, which occurs with incipient hydrolysis, from the low-viscosity PAN emulsions to the high-viscosity, water-swollen gelatinous state, a continuous implementation of the processes that have been described is not practicable in conventional stirring apparatus. Special apparatus is required for the continuous processing of such gelatinous reaction mixtures. In patent application EP-A-783 005 a process is described for the continuous production of superabsorbent polymers, in which aqueous emulsions of crosslinked or uncrosslinked polyacrylonitrile homopolymers and/or copolymers are hydrolysed in a mixing and kneading, long-retention-time reactor (xe2x80x9cList reactorxe2x80x9d) which operates continuously for the implementation of high-viscosity reactions by reaction with aqueous alkali-hydroxide solutions at 70 to 100xc2x0 C., and from the highly viscous gels which form in this process the superabsorbent polymer precipitates out continuously in the form of easily filterable powder as a result of continuous precipitation with low-boiling monoalcohols (ethanol, methanol). After drying and grinding to the desired particle-size range, the finished superabsorber is then obtained. Such an apparatus is technically demanding in comparison with conventional reactors. The object, therefore, was to find an alternative, clearly simplified process.
It has now been found that superabsorbent polymers can be produced from PAN emulsions continuously in conventional reactors if the hydrolysis of PAN emulsions is carried out in an aqueous-alcoholic medium with the removal of ammonia. The alcohol serves as a diluent for the highly viscous intermediate stage which arises in the course of the hydrolysis as well as a precipitating agent. In the course of hydrolysis the fine-particle emulsions based on polyacrylonitrile pass over into an easily stirrable suspension. As a result, it becomes possible to implement the hydrolysis stage continuously in the form of a well stirrable suspension in a cascade of stirred tanks.